Blow molding is a manufacturing process by which hollow plastic parts (such as containers) are formed. It is a process used to produce hollow objects from thermoplastic. The blow molding process begins with melting down the plastic and forming it into a parison or preform. A parison is a tube-like piece of plastic with a hole in one end in which compressed air can pass through. A preform is a molded shape that is prepared prior to the actual blow molding step and then heated to a softened state for blow molding purposes. The basic process has two fundamental phases. First, a parison (or preform) of hot plastic resin is created. Second, a pressurized gas, usually air, is used to expand the hot parison or preform and press it against a mold cavity formed by mold halves. The pressure is held until the plastic cools. Once the plastic has cooled and hardened the mold halves open up and the part is ejected. The mold halves usually have internal cooling channels and/or heating elements to cool or heat the mold as necessary at the required locations.
When mold halves are closed together during the molding process they pinch portions of the parison together to form seals at “pinch-off” areas, but excess parison material at the pinch-off areas is not completely cut away. Protruding edges are cut nearly through, creating an airtight closure by pinching the parison along the tail, shoulder and handle areas which makes it easy to break off or otherwise remove the excesses pieces. One method of obtaining more uniform welding lines is to build “dams” into mold halves at the parison pinch-off areas. These dams force some of molten resin back into the mold cavities to produce stronger weld lines. Mold halves are also required to provide features such as threaded necks that receive caps, and “push-ups” (hollows formed in the base of the containers to provide stability). Because, the mold halves at the pinch-off and feature areas are subjected to comparatively high pressure and mechanical stress, wear damage typically occurs at these areas.
Aluminum alloys are typically used to make blow mold halves due to their good thermal conductivity, light weight and ease of machining. However, aluminum alloys are usually soft and have relatively inferior wear resistance. In order to extend mold life, inserts made of hard and tough metals (typically, beryllium-copper or hardened steel) are used at the pinch-off area as well as areas that provide special features in the aluminum mold (FIG. 1). However, these inserts have to be machined separately from and fastened onto the mold halves, which significantly increases the complexity of the mold design and increases production cost and time for manufacturing and assembling of these inserts. Due to the addition of these inserts, cooling channels have to be designed beneath the inserts, which may restrict the effectiveness of the cooling. In addition, beryllium-copper material typically used to make these inserts is quite expensive and more difficult to machine.
One-piece blow mold halves that eliminate insert segments (FIG. 2) simplify mold design, as they reduce the effort expended to ensure that the various inserts align properly with one another upon assembly. One-piece molds may be constructed relatively quickly, as compared to molds with insert segments, and at low cost. Heat transfer performance is also enhanced over segmented molds, as thermal breaks formed by the junctions of aligned parts are eliminated.
Methods are known that use explosive cladding (or roll cladding, diffusion bonding, etc.) to metallurgically bond a very hard metal layer (such as steel, titanium, etc.) to a softer but very thermally conductive metal substrate (such as an aluminum alloy) to produce blow molds. One problem with technologies such as these is that the layer of very hard metal has different thermal properties than the substrate leading to cracking, especially under prolonged usage. Extra layers of other thermally conductive material may be employed to mitigate against cracking, but this complicates the process and does not satisfactorily address the cracking problem.
There remains a need in the art for further improvement to molds and molding methods.